For example, bone samples can be contaminated by the presence of limestone or organic acids in the soil (like humic or fulvic acids) where the bones were found.
Radiocarbon dating, which is used to calculate the age of certain organic materials, has been found to be unreliable, and sometimes wildly so - a discovery that could upset previous studies on climate change, scientists from China and Germany said in a new paper.
"The only way to control climate change is through a drastic reduction in global CO2 emissions," she warned.
However an earlier study by Dr Wolfgang Knorr, from the University of Bristol, found no evidence the world is losing its ability to absorb carbon.
Although his results showed no difference for now, he said the oceans may lose their ability to soak up carbon in the future.